The present invention relates to a new process for the preparation of 2-mercaptobenzothiazole.
2-mercaptobenzothiazole was described for the first time by A. W. Hofmann in the year 1887. Today, the compound, as well as its derivatives, the so-called "mercaptos", is made in large quantities and is one of the most important vulcanization accelerators. Numerous syntheses and modes of formation are known. The most important preparation processes are based on o-nitrochlorobenzene, hydrogen sulfide and carbon disulfide, or on aniline, sulfur and carbon disulfide (Ullmann, Encyclopedia of Industrial Chemistry, 3rd edition (1960), Urban & Schwarzenberg, publishers, Munich - Berlin, vol. 12, p. 304).
According to the older, discontinuous process, o-nitrochlorobenzene is first reacted with excess sodium hydrosulfide at about 100.degree. C. to form 2-aminothiophenol. Subsequently, the reaction mixture is cooled, mixed with excess carbon disulfide, and again heated to about 80.degree.-90.degree. C. After cooling, it is acidified with sulfuric acid to precipitate the desired 2-mercaptobenzothiazole and the crude product is purified via the calcium salt. In the foregoing process, the yield is about 85% of theoretical.
According to the more recent, continuous process for the industrial preparation of 2-mercaptobenzothiazole, aniline is reacted with a solution of sulfur in carbon disulfide at an elevated temperature (250.degree.-285.degree. C.) and under a pressure of about 150 bar. In such a process, the cyclization reaction is the step determining the velocity and it requires drastic conditions and the use of catalysis (e.g. phosphorus or mercury and iodine compounds). As a result of the reaction conditions, the resulting crude mercaptobenzothiazole is contaminated with tar-like by-products and must be purified through dissolution and subsequent precipitation with sulfuric acid. The yield of pure mercaptobenzothiazole is only about 80% of theoretical.
The concomitant use of a nitro or a nitroso compound, such as nitrobenzene, in the preparation of 2-mercaptobenzothiazole is also known. In the process described in U.S. Pat. No. 2,001,587, aniline, carbon disulfide and nitrobenzene are heated for 6 hours at 220.degree. C. in an autoclave. The carbon disulfide reacts with the aniline to form an addition product, and the nitrobenzene is needed instead of elemental sulfur for the cyclization reaction, by which it is concurrently reduced to a nitrogen compound with a lower valency level. The reaction mixture is subsequently dissolved in sodium hydroxide, filtered, and mixed with hydrochloric or sulfuric acid, which precipitates the 2-mercaptobenzothiazole. Pursuant to the process of USSR Pat. No. 179,306 (C. A. 65, 2268 f (1966), aniline, sulfur, nitrobenzene and carbon disulfide are reacted with one another in a melt at elevated temperature and elevated pressure. In the synthesis, 2 mols of nitrobenzene are used per 5 mols of aniline but, in contrast to the process pursuant to the present invention, the nitrobenzene is not utilized as the only initial aromatic compound and cyclization component, but as an oxidizing agent. In the process, the selectivity and yield are low and the reaction times are very long.
A process for the preparation of 2-mercaptobenzothiazole has now been found, which is distinguished from prior art processes by milder reaction conditions, shorter reaction times and higher selectivities and yield.